1. Field of the Invention
The present invention relates to a lithography technology, and more specifically, to a lithography apparatus and a method for measuring alignment marks.
2. Description of the Related Art
Semiconductor device manufacturing technology has grown rapidly with the development of recent information and computer technologies. The semiconductor IC devices are developed with a goal of higher integration, greater miniaturization and higher speed, and thus the needed specifications for micro-fabrication technology, such as lithography techniques for improving the integrity of the IC devices, has become severe.
The lithography technology is a photo technology that transfers patterns of geometric shapes in a mask (better known as reticle) to a layer of radiation-sensitive material (i.e., photoresist material) covering the surface of a semiconductor wafer. The lithography process includes deposition of the photoresist material, a soft-baking step, an alignment and exposure step, a Post Exposure Bake (PEB) step and a development step.
Apparatuses for the exposure include a stepper and a scanner. The stepper widely used since 1990s exposes a single shot and moves the wafer by one shot along X and Y axes for the subsequent exposure. The stepper, which usually defines a shot area with a mask size of 5 to 6 inches has good uniformity, and light passing through a projection lens exposes the surface of wafer with reduced size of ⅕. The scanner is broadly employed these days because it can further improve the uniformity by exposing with slits within a field and implement large scale field. In such a scanner, the mask size is normally 6 inches and ¼ reduction exposure is performed.
Photoresist materials have resistance to etching of lower layers and are sensitive to radiation, which alters their chemical properties sufficiently so that a pattern can be delineated in them. The photoresist is divided into two: a positive photoresist and a negative photoresist. After irradiation, the positive photoresist in the exposed pattern area absorbs energy, changes its chemical structure, and transforms into a more soluble species. Upon developing, the exposed areas are expunged. In high integration semiconductor fabrication processes, the positive photoresist is used because of its strong etch resistance and high resolution properties. Whilst, the negative photoresist are polymers combined with a photosensitive compound. Following exposure, the photosensitive compound absorbs the radiation energy and converts it into chemical energy to initiate a chain reaction, thereby causing crosslinking of the polymer molecules. The cross-linked polymer has a higher molecular weight and becomes insoluble in the developer solution. After development, the unexposed portions are removed.
One of parameters that determine the performance of a lithographic exposure is registration, which is a measure of how accurately patterns on successive masks can be aligned or overlaid with respect to previously defined patterns on the same wafer. The exposure equipment has to align the mask and wafer before exposing. For the alignment, the equipment drives a stage on which a semiconductor wafer is placed along X-axis, Y-axis and θ-axis (rotation axis) to carry out correction operation, and performs the exposure. The alignment mark refers to labels or marks formed in a predetermined region of the wafer for the alignment of the mask and wafer.
Overlay accuracy is a measure of aligning one layer of a process stack to the subsequent layer, and affected by errors that may occur during the process or from errors in mask or equipment. Typically, patterns for measuring the overlay and alignment marks are formed within the scribe lane of a semiconductor wafer.
The alignment mark lies below the photoresist layer and hence a light source different from the exposure light is used for the alignment mark for preventing the exposure of the photoresist material.
FIG. 1 shows the construction of related art equipment for measuring the alignment mark.
Referring to FIG. 1, light from a light source 100 such as He—Ne laser reflects from or passes through reflection plate 101, optical fiber 102 and lens 103, propagates through a photoresist 140 on a semiconductor wafer or substrate 120, and is reflected backwards from alignment marks 130. The reflected light again passes through the lens 103, and is reflected by a reflection plate 104 to be incident to a CCD (Charge Coupled Device) camera 105. The CCD camera detects and acquires signals of the alignment marks by the use of the light reflected from the substrate.
As explained, the related art equipment for measuring the alignment mark employs a separate light source of He—Ne laser for the alignment mark measurement, which makes the exposure equipment complex and causes an increase of the equipment cost. Moreover, replacing time and cost for the additional light source are inevitable.
Dispersion of light can bring into play the dependence of refraction on the wavelength of light. The He—Ne laser has a wavelength of 633 nm, which is different from the exposing wavelength (for instance g-line of 436 nm, i-line of 365 nm, DUV (Deep Ultra-Violet) of 248 nm or 193 nm) and therefore the refraction becomes different in the exposure process than in the alignment mark measuring process. This difference of refraction may lead to correction errors and result in overlay errors and failure of pattern transfer.